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Guru
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End of Dark Matter in Sight?

12/19/2006 11:40 PM

In todays mainstream cosmology, dark matter is needed to allow clumping of ordinary matter to form before the expansion of the universe makes it impossible to happen.

Writes Physicsweb (1): "In recent years there has been growing support for alternative theories of gravitation to general relativity that do away with the need for dark matter altogether. One of these, devised by Jacob Bekenstein in 2004 at the Hebrew University of Jerusalem, uses vector and scalar fields in addition to the tensor used in relativity, hence the name "TeVeS" (Tensor Vector Scalar). TeVeS has already been shown to explain galaxy dynamics without the need for dark matter. But now, building upon the numerical studies by Pedro Ferreira and colleagues performed earlier this year, Scott Dodelson and Michele Liguori from Fermilab in the US have confirmed that TeVeS can also provide such sustained plasma fluctuations." See figure: "Powering galaxy formation.

Correctly describing the "cosmic power spectrum", which quantifies the temperature anisotropies in the cosmic microwave background radiation, is one of the main challenges any alternative theory of gravitation must face. Without dark matter, the prediction of general relativity (red line) fails to match the observed data (black dots) in either shape or amplitude.

Crucially, general relativity does not give sufficient "power" at any frequency to reach the top of the graph – the threshold for the formation of any structures in the universe. Dodelson and Liguori have shown that TeVeS, a modification of general relativity, does provide the necessary power to reach this threshold. (Data taken from the Sloan Digital Sky Survey.)

Original source: Phys. Rev. Lett. 97 2006

It may a bit early to jump and shout "Eureka", because just the other day dark matter has been explicitly detected right where it should be! See NASA Announces Dark Matter (2)

Links:

(1)http://physicsweb.org/articles/news/10/12/11/1 (Free subscription required)

(2)http://cr4.globalspec.com/thread/2255.

Jorrie

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The Engineer
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#1

Re: End of Dark Matter in Sight?

12/20/2006 5:16 PM

Jorrie,

I almost posted this same article before I saw yours. Do you think its plausible? It sounds possible, but you have a much better understanding of General Relativity than I do. Is your gut telling you anything on this? Are you intrigued or doubtful? Is there a possible extension that might provide insight to dark energy? (Yes I know these are incredibly unfair questions). I do remember the article that provided proof of dark matter, that does seem to contradict this new theory, but maybe that is more an indication of little I know regarding this new theory.

Roger

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Guru
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#3
In reply to #1

Re: End of Dark Matter in Sight?

12/21/2006 11:48 AM

Hi Roger, TeVeS is a relativistic variation of the Modified Newtonian Dynamics (MOND) theory and is highly fashionable among physicists at present. I do not understand it well, but it modifies relativistic (and Newtonian) gravity at large distances/scales, effectively making gravity stronger, meaning it falls off at less than with the inverse square of distance at large distances.

Dr Benoit Famaey of the Free University of Brussels (ULB) commented "It is possible that neither the modified gravity theory, nor the Dark Matter theory, as they are formulated today, will solve all the problems of galactic dynamics or cosmology. The truth could in principle lie in between, but it is very plausible that we are missing something fundamental about gravity, and that a radically new theoretical approach will be needed to solve all these problems. Nevertheless, our formula is so attractively simple that it is tempting to see it as part of a yet unknown fundamental theory. All galaxy data seem to be explained effortlessly".

I have not actually seen the "attractively simple" formula, but I think it will only be true for a physicist working in the field!

Lastly, I do not know about an extension into dark energy! Stronger gravity at large scales still requires dark energy to drive the (presumably) accelerated expansion of the cosmos.

Jorrie

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The Engineer
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#5
In reply to #3

Re: End of Dark Matter in Sight?

12/21/2006 2:40 PM

I don't know, but I look forward to the day when I can spend a little time learning about this.

When dealing with electric fields, you can expand the electric field into the sum of a monopole contribution, dipole contribution, quadrupole contribution, octopole contribution,.... etc. where the monopole is a scalar, dipole is a vector, quadrupole is a tensor rank 3, octopole is a tensor rank 4... etc. The monopole field falls off r2, the dipole field falls off r3, the quadrupole field r4, ... etc. (potentials falling off by r, r2, r3, ....etc.)

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/elequad.html#c2

So at large distances from a charge configuration one needs only to consider the monopole portion (the rest are too small to matter because they drop off much more quickly). At closer distances the dipole contribution must be considered, even closer the quadrupole contribution....etc.

I suspect there is a similarity between this new proposal for gravity and this expansion technique for electric fields, but I won't know for sure till I have the time to read it. It would explain why they say the scalar and vector effects are only apparent at very large distances, though it is clearly different from the electric field expansion in that the vector and scalar contributions seem to disappear at short distances, which is not intuitive to me.

Like I said, I'm just wasting time here with conjecture till I read about it.

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#6
In reply to #5

Re: End of Dark Matter in Sight?

12/25/2006 12:15 AM

In-Reply-To Message by Roger Pink: "When dealing with electric fields, you can expand the electric field into the sum of a monopole contribution, dipole contribution, quadrupole contribution, octopole contribution,.... etc. where the monopole is a scalar, dipole is a vector, quadrupole is a tensor rank 3, octopole is a tensor rank 4... etc. The monopole field falls off r2, the dipole field falls off r3, the quadrupole field r4, ... etc. (potentials falling off by r, r2, r3, ....etc.)... I suspect there is a similarity between this new proposal for gravity and this expansion technique for electric fields,... It would explain why they say the scalar and vector effects are only apparent at very large distances,..."

It sounds like a good theory, Roger. I think you're on the right track. Einstein modified some of his equations for this, and later said it was his greatest blunder because he had not realized that they indicated the expansion of the universe. Maybe the equations need to be modified again, but in a different way. A similar situation exists in the atom. The particles need to be maintained at a certain distance from each other - not too close, and not too far away.

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#2

Re: End of Dark Matter in Sight?

12/21/2006 8:14 AM

While one experiment has detected dark matter there is a whole lot of verification that has to go on before a conclusion can be drawn. There will also be debate on how much dark matter actually exists.

In the meantime the concept of variable gravity has one important follow on effect: if gravity is variable could it possibly be artifically modified for a useful purpose? The mind boggles at the opportunities.....

Eric

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Guru
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#4
In reply to #2

Re: End of Dark Matter in Sight?

12/21/2006 11:54 AM

Hi Eric, you wrote: "... if gravity is variable could it possibly be artificially modified for a useful purpose? The mind boggles at the opportunities....."

The only problem I see with your suggestion is that the mind also boggles at the galactic proportions of the scale that these modification seems to become useful at!

Jorrie

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#7

Re: End of Dark Matter in Sight?

12/29/2006 6:25 PM

I guess what I would most like to say to everyone, is you all have a wonderful happy, healthy, and prosperous 2007.

Then I started looking at that hyper-mathematics, and started to feel like the dormouse, in the Mad Hatter's Tea Party with Alice in Wonderland. I kept falling asleep. Mostly because there was a factorial 'K!' in the position of a divisor in binomial expansion, and my little brain said what is a 'k' and are not factorials very very very large occasionally, and something divided by something very big is usually very tiny?..... Paul Erdos love little epsilons, and always was photographed with a boss epsilon on his lap. often sitting with a slave epsilon next to him. Will we get to read from the Supreme Fascist's Book on the subject heading 'Dark Energy'? ...We will have to enquire from the Mad Hatter, he says the answer is 10 shillings and sixpence. The hat comes gratis. It is covered with 'Dark Matter' a fine brushed silk velvet.... More tea CR4 Readers? ......We shall wait and see, is my advice.

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Guru

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#8
In reply to #7

Re: End of Dark Matter in Sight?

12/30/2006 10:45 AM

Alastair responds yet again to Alastair. The dormouse has had a good kip, and dreamt of wondrous expanded surfaces. Dreams are hard to interpret, but sometimes a picture tells a thousand words.

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Anonymous Poster
#10
In reply to #8

Re: End of Dark Matter in Sight?

05/16/2007 7:40 PM

"Alastair responds yet again to Alastair"???

I remember you. You are the wacko from "wacko science", aren't you? Got your medication today, before allowed to write again?

- H/TLV

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Guru

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#9

Re: End of Dark Matter in Sight?

12/30/2006 10:51 AM

Fractal Magic & Strange spiral stairways down the Rabit's hole. "No Time No Time" He says looking to his Watch.

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